1. Field of the Invention
The present invention relates to a surface acoustic wave device. In particular, the present invention relates to a surface acoustic wave device including an IDT electrode of which a portion is embedded in a groove located in a piezoelectric substrate and a dielectric film arranged so as to cover the IDT electrode.
2. Description of the Related Art
In recent years, surface acoustic wave devices such as surface acoustic wave resonators and surface acoustic wave filters which utilize surface acoustic waves have been widely used.
A surface acoustic wave device includes a piezoelectric substrate and an IDT electrode formed on the piezoelectric substrate. In the surface acoustic wave device, an elastic wave excited by the IDT electrode propagates as a surface acoustic wave on the surface of the piezoelectric substrate.
In the surface acoustic wave device, a LiNbO3 substrate, a LiTaO3 substrate, or the like is generally used as the piezoelectric substrate. The LiNbO3 substrate and LiTaO3 substrate have negative temperature coefficients of frequency (TCF). Thus, when the LiNbO3 substrate or the LiTaO3 substrate is used as the piezoelectric substrate, the frequency characteristics of the surface acoustic wave device change with temperature change. In view of this, for example, WO 2010/016192A1 described below states that a portion of an IDT electrode is embedded in a groove formed on a piezoelectric substrate and a SiO2 film having a positive TCF is further formed so as to cover the IDT electrode, thereby reducing the temperature dependency of the frequency characteristics of a surface acoustic wave device.
In addition, in a surface acoustic wave device, a dielectric film may be formed on a piezoelectric substrate so as to cover an IDT electrode, for the purpose of improving voltage resistance and other purposes unlike improvement of frequency-temperature characteristics as described above.
However, when a dielectric film is formed on a piezoelectric substrate so as to cover an IDT electrode, favorable frequency characteristics may not be obtained.